UPGRADE YOUR SKILLS: Learn advanced Swift and SwiftUI on Hacking with Swift+! >>

Sequences get prefix(while:) and drop(while:) methods

Available from Swift 3.1

Paul Hudson      @twostraws

Two useful new methods have been added to the Sequence protocol: prefix(while:) and drop(while:). The former returns the longest subsequence that satisfies a predicate, which is a fancy way of saying that you give it a closure to run on every item, and it will go through all the elements in the sequence and return those that match the closure – but will stop as soon as it finds a non-matching element.

Let's take a look at a code example:

let names = ["Michael Jackson", "Michael Jordan", "Michael Caine", "Taylor Swift", "Adele Adkins", "Michael Douglas"]
let prefixed = names.prefix { $0.hasPrefix("Michael") }

That uses the hasPrefix() method to return the subsequence ["Michael Jackson", "Michael Jordan", "Michael Caine" – the first three elements in the sequence. It won't include "Michael Douglas", because that comes after the first non-Michael. If you wanted all the Michaels regardless of their position, you should use filter() instead.

The second new method, drop(while:) is effectively the opposite: it finds the longest subsequence that satisfies your predicate, then returns everything after it. For example:

let names = ["Michael Jackson", "Michael Jordan", "Michael Caine", "Taylor Swift", "Adele Adkins", "Michael Douglas"]
let dropped = names.drop { $0.hasPrefix("Michael") }

That will return the subsequence ["Taylor Swift", "Adele Adkins", "Michael Douglas"] – everything after the initial Michaels.

BUILD THE ULTIMATE PORTFOLIO APP Most Swift tutorials help you solve one specific problem, but in my Ultimate Portfolio App series I show you how to get all the best practices into a single app: architecture, testing, performance, accessibility, localization, project organization, and so much more, all while building a SwiftUI app that works on iOS, macOS and watchOS.

Get it on Hacking with Swift+

Sponsor Hacking with Swift and reach the world's largest Swift community!

Other changes in Swift 3.1…

Download all Swift 3.1 changes as a playground Link to Swift 3.1 changes

Browse changes in all Swift versions

Unknown user

You are not logged in

Log in or create account

Link copied to your pasteboard.